In restraint systems, for example in automobiles, it should be ensured for the proper operation of safety devices, such as front, knee, side and/or head airbags, that a restraint system provided in the motor vehicle is also used by a passenger and is also properly locked when the same is being used. Otherwise, the safety devices, such as airbag devices, could even lead to injuries of the affected passenger in the case of a collision. It has already been proposed to check the locking state of a latch of a seat belt inserted into a seat belt lock. From knowing the locking state of the seat belt, for example, signals can be generated to alert the passengers by way of a signal that they should put on and latch the seat belt. Ever since the introduction of airbag devices, the information on the locking state of the seat belt system has also become relevant to the activation or deactivation of mechanisms for inflating driver and passenger airbags or knee, side and head airbags.
Hall sensors are widely used as proximity switches or as sensors for non-contact determination of the state of components, which can occupy two positions. In principle, Hall sensors include (e.g., consist of) a semiconductor layer that is provided with constant current, such as in an integrated design. The constant current is influenced by a magnetic field component perpendicular to the semiconductor layer, and the sensor yields a Hall voltage that can be analyzed, that can be tapped off and that can be used to analyze a state or else can be employed directly as turn-on voltage. The integrated design of Hall sensors offers the possibility of integrating on the Hall sensor an analysis circuit that is suitable for analyzing the switch state.
From EP-A-0 861 763, a seat belt lock with an integrated prestressed Hall sensor is known, which detects, in a non-contact manner, the state of a locking body or an ejector for a lock latch introduced into the seat belt lock. In this case, a Hall sensor is arranged with a Hall field in the immediate vicinity of a permanent magnet. By a change in position of the locking body or the ejector, which to this end includes (e.g., consists of) a ferromagnetic material, the magnetic field of the permanent magnet is changed. As a result, the signal of the Hall signal is changed, and at the output of the Hall sensor, the change in state can be tapped off as a change in voltage. In an alternative variant embodiment, it is proposed to install the Hall sensor with a Hall field without a permanent magnet and for this purpose to design the locking body or the ejector as a permanent magnet. Also, in this arrangement, the change in position of the locking body or the ejector is to be detectable by a change in the Hall voltage.
In the seat belt lock according to EP-A-0 861 763, the Hall sensor is to be positioned very carefully relative to the locking element or the ejector. A subsequent incorporation of the Hall sensor can therefore be relatively labor-intensive and costly. In addition, the Hall sensor is relatively sensitive compared to outside scatter fields, which can even be caused by, for example, a magnetic key attachment. Optionally, even an additional shield should be attached, which can further complicate the design or incorporation. The susceptibility relative to outside scatter fields is also still magnified in that the signal changes—because of the comparatively short distances that are covered by the locking body or the ejector during the closing or opening of the seat belt locking—are relatively small. Also, the seat belt lock variants without prestressed Hall sensors, in which either the locking body or the ejector is designed as a permanent magnet, have proven to be not very practicable. The signal changes that can be achieved are also relatively small here. By the vibrations of the locking body and the ejector when the seat belt lock is opened and closed, a demagnetization can result over time. This can ultimately lead to the fact that the Hall sensor is ineffective, and the changes in state of the seat belt lock can no longer be detected.
To reduce fuel consumption, major efforts are being undertaken to reduce the weight of inserts and attachments of motor vehicles. To this end, inserts, such as, e.g., seat belt locks of restraint systems, are also made smaller. Because of the smaller space available, the devices for detecting the locking state of the seat belt locks should also be modified. With respect to the desired robustness of the devices, to ensure up to, for example, 100,000 cycles of operation, such modifications are not trivial.